Propulsion systems propel objects, such as rockets and missiles. Liquid systems and solid systems are the two basic types of propulsion systems that are most generally used. In a solid propulsion system, solid rocket fuel and an oxidizer are mixed together and allowed to cure inside a rocket case to form a solid propellant material, which is then ignited in the rocket case. Upon ignition, pressure forms within the rocket case and gases are released through a nozzle to produce thrust. In a solid propellant system, the solid propellant burns uninterrupted until all the propellant is exhausted.
In a liquid system, a liquid oxidizer is fed into a combustion chamber with a liquid fuel. The oxidizer and liquid fuel are mixed in the combustion chamber, where they react to produce gases under high temperature and high pressure. The gases exhaust through a nozzle from the combustion chamber to thereby produce thrust.
Another type of propulsion system is the hybrid system, which are generally not as widely used as liquid and solid fuel systems. A hybrid system combines aspects of both liquid systems and solid systems in that one propellant is stored as a solid and another propellant is stored as a liquid. In a typical hybrid system, the solid material is used as the fuel and the liquid material is used as the oxidizer.